1. Field of the Invention
This invention relates generally to plasma treatment of material, and in particular to an apparatus for continuous treatment of continuous material with a plasma of a plasma gas under vacuum. The invention is applicable to the treatment of photographic support material, whereby transient surface modifications induced by plasma treatment are exploited to enable improved adhesion and coatability of photographic emulsions to the support. The support is particularly a polyethylene coated paper photographic support. It will be convenient to hereinafter describe the invention in relation to this exemplary application, although it is to be appreciated that the invention is not limited to that application.
2. Description of the Related Art
Plasma or glow discharge treatment of material surfaces has-been reported in numerous publications including J. R. Hollahan and A. T. Bell, "Techniques and Applications of Plasma Chemistry", Wiley, N.Y. 1974 and H. Yasuda, J. Macromol. Sci. Chem. A10,383 (1976). Such treatment has also been the subject of a variety of patents including U.S. Pat. Nos. 3,057,795, 3,288,638, 3,475,307, 3,526,583, 3,761,299, 3,837,886, 3,944,709, 4,437,324, 4,451,497, 4,457,145 and 4,551,310, and British Patents 997093 and 1579002.
The discharges occuring during treatment may be struck in most gases using voltages from several hundred volts to several kilovolts. Pressures are usually in the range 1.33 to 1333.22 pascal (0.01 to 10 Torr) and discharge frequencies range from DC to 100 MHz. In most cases of interest here, the treatment affects the surface properties of materials by modifying the chemical nature of the surface layer. Plasma treatment may be used to improve the bonding of coatings such as adhesives, dyes, inks, polymers and photographic emulsions, to improve the wettability of hydrophobic materials or to alter mechanical properties such as the coefficient of friction or the cohesiveness of textile fibres. In another variation, thin polymer films may be deposited on most surfaces by plasma polyermization from a discharge in an appropriate gaseous monomer. Apart from their potential use as bonding layers, such thin films find other possible uses including protective and decorative layers, optical coatings, capacitor dielectrics and semipermeable membranes.
Few of the reported applications of plasma treatment have utilized any short-lived properties (lasting for up to several hours) of the treated surfaces in subsequent processes. It has been shown, for example in British patent 997093, that surface radicals are produced by plasma treatment of a wide range of organic materials and can be used to initiate graft polymerization if the treated surface is exposed to monomer vapor a short time after treatment. Studies have also shown that surface hydrocarbon radicals are rapidly destroyed in most cases by exposure to oxygen but allylic, polyenyl and peroxy radicals which can be formed in this process may survive for time periods longer than several minutes. The prior art literature does not appear to contain any reference to the possibility of using such short-lived species to react with coatings or films which are rapidly applied to a treated surface. These applications would appear to be of a different nature than those of graft polymerization in that radicals are not used to initiate polymerization but are merely allowed to react with molecules brought adjacent to the surface.
In the photographic industry, the wetting and adhesive properties of hydrophobic (unsubbed) photographic support such as polyethylene coated paper has traditionally been improved prior to coating by subjecting the support to Corona Discharge Treatment. The corona can be formed in air at atmospheric pressure by subjecting an electrode above a dielectric covered roller to a high voltage (15 KV peak) low frequency (10 KHz) signal.
As well as improving the wetting by, and the adhesion to, aqueous photographic emulsion layers, corona discharge treatment introduces strongly bound polar charge into the surface of the support. This charge can disturb the liquid emulsion at the point of application to create non-uniformities in the coated layer. To reduce the severity of this, discharge equipment is included between the corona treater and the coating station. Conventional electrostatic dischargers, such as stressed wire corona dischargers, multiple point corona bars, air jet ionizers and radio-active dischargers are not wholly effective because the polar charge produces a very small external field and the number of ions produced per unit time is insufficient to meet the demand. As will be shown later, plasma treatment can in fact remove non-uniform polar charge.
The deposition of a uniform layer of polar charge on photographic support is of importance in high speed coating applications. The polar charge attracts the coating bead which would not otherwise wet the support at high coating speeds, thereby eliminating skottle (air entrainment which disturbs coating uniformity). Polar charge layers with potentials up to 3000 V have been deposited by V-channel ionizers or polar charge brushes. These devices essentially create a corona discharge above the web surface and simultaneously transport charge to the surface of the web. The achievement of controlled uniform levels of polar charge requires a considerable level of complexity in these devices. As will be shown later, a uniform polar charge layer may be deposited in a similar but simpler fashion from a glow discharge during plasma treatment of a photographic support.
Most of the applications of plasma treatment disclosed in the above patents utilize a batch process. That is, the material to be treated is placed inside a vacuum chamber which is then evacuated and the treatment carried out. However, where continuous webs or filaments are to be treated it may be more cost effective to transport the material through a vacuum seal into the treatment zone and back out again in a continuous manner. This possibility is addressed in a number of patents. In that regard, moving filaments or rods can generally be sealed in a satisfactory manner using slot seals as disclosed in U.S. Pat. Nos. 3,502,499 and 3,952,568, and British patent 1238450. Moving webs can be sealed by slot seals as discussed in U.S. Pat. Nos. 3,057,795, 3,179,482, 3,761,299, 4,026,787 and 4,072,769, liquid seals as discussed in U.S. Pat. No. 3,870,610, press seals as discussed in U.S. Pat. Nos. 3,384,900, 3,867,768 and 4,065,137, roller seals as discussed in U.S. Pat. Nos. 3,205,086, 3,246,365, 3,384,900, 3,867,768, 4,048,953, 4,065,137, 4,179,820 and 3,334,908, and belt seals as discussed in U.S. Pat. No. 4,048,953.
Slot seals, liquid seals and press seals have all been utilized in apparatus involving plasma treatment of webs.
However, slot seals are unsatisfactory due to their high leak rates and hence large pumping capacity requirements. Web flutter can also be a problem and may cause web damage. Liquid seals are cumbersome and may produce undesirable contamination of many substrates.
Press seals can provide good vacuum sealing but could cause frictional damage to web surfaces. Webs of high tensile strength are also required to pass satisfactorily through press seals so that they are not suitable for the present exemplary application.
Roller seals have been used in various apparatuses for transport of webs or sheets into and out of a vacuum, although they have not been used for plasma treatment applications. Generally, the roller seals have incorporated non-compliant rollers and have been used to transport webs of high tensile strength (e.g. metal strips and wood veneer) or deformable webs (woven fabrics). Non-compliant rollers allow leakage of air in the gaps defined between the edges of the web and the edges of the rollers. Furthermore, they do not readily allow control of the pressure exerted by the rollers on the web. Control of the roller to web pressure is critical for the avoidance of steering and creasing problems when transporting more fragile webs, such as photographic supports. U.S. Pat. No. 3,334,908 describes apparatus incorporating compliant roller seals for sealing about high tensile strength webs or sheets. However, in that patent a non-compliant backing roller engages with the compliant sealing rollers to define nips, and a series of non-compliant wiper rollers back onto the sealing rollers to seal off leakage that may occur because of the necessary clearance between the sealing rollers and their housing. These multiple wiper rollers again make it difficult to control the sealing roller to web pressure. In addition, lack of appropriate adjustment in the wiper rollers does not permit the maintenance of minimal nip pressure to be applied to the transported web.
U.S. Pat. No. 4,048,953 describes a continuous belt seal for transport of fragile webs (papers) through a vacuum system, although again, not for plasma treatment applications. This apparatus is relatively complex and operates at a maximum web speed of 18 meters per minute, which is much slower than current coating speeds of most photographic products.
In European Patent Application 0,337,369, a continuous vacuum deposition apparatus is described that includes seal devices made of a set of three pinch rolls. The three pinch rolls are arranged in parallel on one plane and a pair of seal bars are positioned on said parallel plane to form one end of the vacuum chamber. For each successive vacuum chamber a set of three pinch rolls is required. In the present invention each successive chamber requires only one additional roll.
French Patent Application 2,485,319 shows a typical vacuum sealing system for treatment of a web with low temperature plasma wherein each sealing chamber is provided with a pair of nip rolls. The present invention reduces the number of rolls required for each sealing chamber.
Finally, in French Patent Application 2,487,696 a vacuum treatment apparatus is described which continuously treats the surface of a plastic molding under vacuum. Each auxiliary vacuum chamber includes a pair of seal rolls. The present invention reduces the amount of seal rolls required for each vacuum chamber.